This invention relates to a method of manufacturing a semiconductor light emitting device such as light emitting diodes or semiconductor lasers, and more particularly relates to a method of manufacturing a semiconductor light emitting device using compound semiconductor crystal having aluminum as a basic material.
In recent years, as applications of semiconductor lasers have been developed, highly efficient mode-controlled semiconductor lasers have been required as a light source for optical fiber communication and for use in reading optical disks such as Digital Audio Disk (DAD), Video Disk and Document File. One of the semiconductor lasers which has been developed for that purpose has a substrate with grooves so as to achieve current confinement effect. Another has a double heterostructure whose upper layer is appropriately processed so that both the current confinement effect and the optical confinement effect might be achieved.
Also, Metalorganic Chemical Vapor Deposition (MOCVD) has been developed and applied to the manufacture of some semiconductor lasers.
A mode controlled heterojunction semiconductor laser which has a current confinement effect and a refractive index guide effect is disclosed in Japanese Provisional Publication No. 60-14482. FIG. 1 shows this type of heterojunction semiconductor laser, in which current is confined only in the stripe portion 10 so that the threshold current at which oscillation starts might be as small as 15 mA.
The light generated in the active layer 16 is also confined in the stripe portion 10 owing to the difference of the refractive index between the stripe portion 10 and its outer portions. Consequently, the longitudinal mode of this semiconductor laser is stable until an output power range of 30 mW during continuous operation is achieved. However, this type of semiconductor laser has disadvantages associated with the manufacturing method described below.
After n-type GaAs substrate 12, n-type Ga.sub.0.6 Al.sub.0.4 As cladding layer 14, undoped Ga.sub.0.94 Al.sub.0.06 As active layer 16, p-type Ga.sub.0.6 Al.sub.0.4 As cladding layer 18, and n-type Ga.sub.0.6 Al.sub.0.4 As current blocking layer 20 are grown successively, the current blocking layer 20 and cladding layer 18 are partially etched away using photolithographic techniques to form stripe portion 10 at the center. Then, p-type Ga.sub.0.7 Al.sub.O.3 As optical guide layer 22, p-type GA.sub.0.6 Al.sub.0.4 As cladding layer 24 and p-type GaAs contact layer 26 are additionally grown on that surface successively by MOCVD.
In this process, the p-type cladding layer 18 is doped with high vapor pressured zinc. The result is that the zinc might evaporate in hydrogen atmosphere when the reactor is heated up in order to grow the optical guide layer 22, the cladding layer 24 and the contact layer 26. Accordingly, the density of the acceptors in the stripe portion 10 becomes lower as shown in FIG. 1 or the surface 30 of that portion 10 may turn to n-type if things come to the worst. The low density of acceptors in stripe portion 10 makes the current concentrate on the circumference 28 of the stripe portion 10 so that it might be damaged by light and heat caused by the current and the lifetime of the semiconductor laser might be shortened.
Further, the surface 30 of the stripe portion 10 turns to an n-type from a p-type, a n-p-n-p thyristor structure is formed with the n-type active layer 16, the p-type cladding layer 18, the n-type surface region 30 of the cladding layer 18 and the p-type optical guide layer 22. As a result, the p-n junction formed between the n-type surface portion 30 of the cladding layer and the p-type cladding layer 18 resists the flow of current until it breaks down. This means that the threshold current of the device becomes higher and the operating characteristic becomes worse as shown in FIG. 8.
Moreover, the carrier density of the contact layer 26 must be above 1.times.10.sup.19 cm.sup.-3 in order to make good contact between the metal contact 32 and the contact layer 26 because the metal contact 32 comprises nonalloys such as Ti/Pt/Au. However, while the temperature of the reactor becomes low after growth of the contact layer 26 is completed, the partial pressure for the zinc decreases so that the zinc might evaporate from the surface of the contact layer 26. Accordingly, as shown in FIG. 1, the surface density of the contact layer decreases and contact resistance increases. An additional step of etching the contact layer 26 by about 1 .mu.m is required in order to decrease the resistance.
In addition, when the additionally grown layers consisting of the optical guide layer 22, the cladding layer 24 and the contact layer 26 are formed, the exposed surface of the cladding layer 18 or the stripe portion 10 may be covered with an aluminum oxide layer. This type of oxide layer would cause a voltage drop between the cladding layer 18 and the optical guide layer 22. Therefore, the device might be heated up. The characteristics of a semiconductor laser are very sensitive to the operation temperature. Namely, a higher temperature is more harmful to the operation of the device. For example, if the operation temperature rises by 10.degree. C., the lifetime of the device is shortened to about a half.